Transport in Phloem
The basic necessities of plants, water, are taken up by the roots. Another purpose served by the roots is to absorb water soluble mineral nutrients from the soil. Mineral nutrients move together with water in long distance pathways provided by the vascular system to reach the entire plant. Roots cannot feed themselves in the darkness of soil.
Several other tissues in the plant body also lack photosynthetic apparatus completely or possess it partly in the sense that they cannot manufacture enough food required to support their life processes. Leaves on the other hand rely on roots for water and minerals but manufacture more food than they actually need. Hence, leaves serve as the source of food for other tissues which may store excess of it so that it can be used by plant for the perpetuation and spread of species.